German Patent 23 05 249 and corresponding U.S. Pat. No. 3,724,733 disclosed a web infeed device in which the tension of a roll or web of paper is kept constant by means of a tensioning roller in the form of a compensating roller. In order to return the compensating roller into a preset initial position after a deflection caused by a breakdown; i.e., a change in the tension of the paper web, a traction roller with a shaft drive is located upstream of the compensating roller. The shaft drive of the traction roller is driven both by the main drive of the printing machine and by a second drive which is a reversible servomotor so that the rpm of the main drive and of the servomotor are cumulatively superimposed on each other by means of the shaft drive. The servomotor is controlled by a control circuit which receives a tension signal, or error signal, which is proportional to the deflection of the tensioning roller from its nominal position. A strain gauge element provides the tension signal and this signal is cumulatively combined with a signal proportional to the speed of the traction roller to provide proportional reinforcement of this signal, the proportional speed-dependent portion of the control circuit being located in the measuring element.
A disadvantage of the device shown in German Patent 23 05 249 is the use of a servomotor which is powered separately from the main drive of the printing machine, which motor must be precisely controlled in its rpm, and which is connected to the cumulative drive. For one thing, this motor and its rpm governor must be so powerful, because of their function, as to entail considerable cost for their acquisition and operation. For another, a motor of the size required for this purpose has the disadvantage that its rotating parts have a large mass which is difficult to reverse because of its physical inertia.
A further disadvantage of devices such as that shown in the German Patent 23 05 249 is the use of a control circuit which, although supplied with a signal proportional to the speed of the traction roller, does not provide a signal which corresponds to the rate of deflection of the tensioning roller which is to be controlled. However, the decisive disadvantage of this device lies in the fact that if the motor, its rpm governor, or the controller becomes inoperative, the traction roller can only be operated at an rpm which is proportional to the speed of the printing machine. As a result, the feed device operates with only one defined roll tension which, depending on the type of paper used, may exceed the required input tension of the printing machine (with thin paper) or may fail to attain the required tension (with heavy paper).